1. Field of the Invention
The present invention pertains to contact lenses, and more particularly to lenses used in the treatment of presbyopic patients.
2. Brief Description of the Prior Art
Contact lenses are now widely used for the correction of many different types of vision deficiencies requiring simpler lens designs and less complicated fitting practices. Contact lenses have also been proposed for more demanding applications, and are now widely used for the correction of many types of vision impairments, including presbyopia, which is of interest here.
Contact lens practitioners generally agree that the most desirable form of correction for the majority of presbyopic patients is a bifocal contact lens which successfully provides correction for both near and distance vision, with minimum fitting problems and minimum chair time. The fitting of presbyopic contact lenses has become a specialized field, requiring special fitting techniques and highly developed skills not required in other types of contact lens practice. Despite various improvements, the fitting of presbyopic contact lenses still requires a disproportionately greater amount of chair time, which adds to the cost and inconvenience associated with treatment.
Several different theories of correction for presbyopic patients have been proposed, each generally associated with different lens design features. The contact lens designer has many variables which can be combined to produce a wide variety of vision corrections. Several of the more prominent variables include centering the lens on the patient's cornea or pupil (central fit), or positioning the lens off-center with respect to the wearer's eye; shifting the near and distant portions of a lens about the wearer's eye in response to a change in the wearer's gaze (alternating vision fit) or fixing the lens at a relatively constant position about the wearer's eye (simultaneous vision fit); adjusting the relative size of the near and distant portions of the lens; and placing either the near or the distant portion of the lens at the center of the wearer's eye. Different theories of correction require a carefully controlled combination of the above variables as well as other design variables not mentioned herein. Considerable skill and experience is required to provide a successful bifocal contact lens design which provides successful vision correction for presbyopic patients, is comfortable to the wearer, and requires a minimum of chair time to fit the lens to meet the specific needs of a particular patient.
One example of a contact lens positioned off-center with respect to the wearer's eye is given in U.S Pat. No. 3,726,587, in which a truncated lens having a prism ballast is provided with an offset bifocal segment for viewing near objects. Due to the additional thickness and weight of the truncated lens and prism ballast, the lens is used only when necessary to correct for astigmatism. Accordingly, the use of the lens is considerably limited. The central portion of the lens for viewing near objects has a size of 0.10 to 0.30 mm less than the near-point pupil size of the wearer's eye. Use of the lens is further limited to patients requiring near and distant correction which is closely related (i.e., within 3X), so that optical jump and fuzziness are avoided.
An area of significant improvement in contact lenses long sought in the art is the reduction of chair time required to fit lenses to presbyopic patients. A major factor in the amount of chair time required for fitting the lens to a patient with presbyopia is the precise alignment that must be maintained between the two focal areas of the lens and the patient's pupil, throughout a wide range of movement of the patient's eye, limbal area and eyelid. Typically, relatively closefitting lenses are employed to maintain a stable position. However, patient discomfort is increased with tighter-fitting lenses, and recent studies have indicated problems of inadequate circulation and cleansing of tears associated with tighter-fitting lenses. Further, these problems are at times aggravated when precautions are taken to limit rotation of a contact lens about a wearer's eye.